Previously, nitrogen rejection from natural gas was confined to a naturally occurring nitrogen content, thus an essentially constant feed composition. Recent methods of tertiary oil recovery utilizing nitrogen injection/rejection concepts, however, necessitate nitrogen rejection units (NRU) that can process a feed gas stream of a widely varying composition because the associated gas from the well becomes diluted by increasing amounts of injected nitrogen as the project continues. In order to sell this gas, nitrogen must be removed since it reduces the gas heating value. These nitrogen rejection processes may incorporate a methane heat pump cycle to provide refrigeration for the process and typically would use conventional heat exchangers to condense the methane gas stream.
Countercurrent heat exchange is commonly used in cryogenic processes because it is relatively more energy efficient than crossflow heat exchange. Heat exchangers of the plate-fin variety which are typically used in these processes can be configured in either a "cold-end up" or a "cold-end down" arrangement. When essentially total condensation of a gas stream is effected one approach is to use the cold-end up arrangement because "pool boiling" may occur in a cold-end down arrangement when one of the refrigerant streams comprises two or more components. Pool boiling degrades the heat transfer performance of the heat exchanger. Therefore a cold-end up arrangement is preferred. The design of such cold-end up exchangers must insure that at all points in the exchanger, the velocity of the vapor phase is high enough to carry along the liquid phase and to avoid internal recirculation, i.e. liquid backmixing which degrades the heat transfer performance of the exchanger.
However, in certain processes, such typical cold-end up heat exchangers are not adequate. There are particular problems in heat exchange situations associated with cryogenic plants for purifying natural gas streams having a variable nitrogen content. One such application in a nitrogen rejection process for which conventional heat exchange technology is inadequate involves incorporating a methane heat pump cycle into a process for treating a natural gas feed stream having a variable nitrogen content. The methane recycle must be essentially totally condensed in countercurrent heat exchange with a multicomponent vaporizing hydrocarbon stream.
As the nitrogen content gradually increases over the years, the inlet and outlet temperatures of the heat exchanger in which the methane recycle stream is condensed change. In addition, the flow rate, pressure, temperature and composition of the vaporizing hydrocarbon stream also change as the feed composition becomes progressively richer in nitrogen. These changes affect the relative positions within the heat exchanger used for cooling, condensing and subcooling the methane recycle stream. Since there is no vapor to carry over the methane liquid after the recycle stream has been condensed, the design of an operative, efficient cold-end up heat exchanger is problematical.
In order to avoid the upward stability problems that are characteristic of cold-end up exchangers, workers in the art have utilized a cold-end down approach. This approach eliminates the difficulty of carrying over the condensed liquid at the various heat exchanger operating conditions. However, the vaporizing streams in the heat exchangers which provide the condensing duty consist of at least one multicomponent hydrocarbon stream that tends to "pot boil" in cold-end down configurations. The "pot boiling" effect tends to warm up the multicomponent stream at the coldest part of the heat exchanger. To overcome this effect, the pressure of this return stream must necessarily be lowered which results in additional compression requirements and increased power consumption.
The changing conditions of the vaporizing multicomponent stream make the design of cold-end down exchangers problematical.
A worker of ordinary skill in the art of cryogenic processes can choose from a host of heat exchangers such as, for example, helically wound coil exchangers, shell and tube exchangers, plate-exchangers and others.
Illustrative of the numerous patents showing heat exchangers having a serpentine pathway for at least one fluid passing in a heat transfer relationship with another fluid are U.S. Pat. Nos. 2,869,835; 3,225,824; 3,397,460; 3,731,736; 3,907,032 and 4,282,927. None of these patents disclose the use of a serpentine heat exchanger to solve the problem of liquid backmixing associated with cold-end up heat exchangers for cooling, condensing and subcooling a methane recycle stream in a methane heat pump cycle of a nitrogen rejection process.
U.S. Pat. No. 2,940,271 discloses the use of two heat exchangers in a process scheme for the separation of nitrogen from natural gas. No mention is made of the problems associated with condensing a substantially single component gas stream against a multicomponent vaporizing hydrocarbon stream.
U.S. Pat. No. 4,128,410 discloses a gas treating unit that uses external refrigeration to cool a high pressure natural gas stream by means of a serpentine, cold-end down heat exchanger. Since the refrigerant extracts heat from the natural gas stream as the refrigerant courses through the serpentine pathway in the heat exchanger, there is no problem with stability in an upwardly condensing circuit.
U.S. Pat. No. 4,201,263 discloses an evaporator for boiling refrigerant in order to cool water or other liquids. The evaporator uses a sinuous path consisting of multiple passes on the water side of the exchanger, in which each successive pass has less area, so that the velocity of the water is increased from the first pass to the last pass.
Serpentine heat exchangers have also been used in air separation processes as a single phase subcooler, that is for cooling a liquid stream to a lower temperature without backmixing due to density differences. Another application involves supercritical nitrogen feed cooling in a nitrogen wash plant over a region of substantial change in fluid density.